This invention relates to improved AC or DC solenoid for operating hydraulic or air valves, or other machines.
When the circular or polygonal plunger type armature of a solenoid is attracted by a stationary core upon application of alternating or direct current to the solenoid coil, the attraction force-stroke characteristics of the solenoid are generally such that if the stroke S is large, the force F is small, while the force increases with a decrease of the stroke, as shown by curve "a" in FIG. 2. The stroke means the distance between the end surfaces of the armature and the stationary core which contact each other when the armature has moved. Known solenoids have the disadvantages that in order to increase the force for a greater stroke to or over a certain level, it is imperative to increase the force for a smaller stroke to an unnecessary degree, resulting in the generation of noise and the shortening of the life of the armature and the stationary core.
Known solenoids have a path of the magnetic field passing through the yoke 8 of a solenoid coil 7, an armature tube 3, an armature 1, a stationary core 2, the armature tube 3 and the yoke 8 as shown in FIG. 1 labeled "Prior Art." When the armature 1 has been attracted by the stationary core 2, it merely follows that the distance S between the end surfaces of the armature and the stationary core is reduced to zero, with no appreciable change in the magnetic path or the effective cross-sectional area thereof, and therefore, the attractive force becomes stronger with a decrease of the stroke.
A solenoid of the armature in tube type as shown in FIG. 1 (Prior Art) is secured to a valve body by engaging with a thread in the solenoid mounting surface of the valve a thread 11 on the stationary core to which the sealed armature tube through which the armature moves is attached in an oil-tight fashion. In known solenoids of this type, the closure member 4 for the free end of the armature tube 24 is, for example, provided with a grip portion 10 for a tool as shown in FIGS. 1 and 1A (Prior Art), to which a spanner or the like is applied to turn the armature tube 24. The torque which can be applied to turn the armature tube 24 is necessarily limited to protect it against distortion. Consequently, oil leakage is likely to occur between the stationary core 2 and the valve 14 because it is difficult to establish an appropriate seal therebetween, while the application of an excessively great force tends to cause deformation of the thin wall 3 of the armature tube 24. An increase of the wall thickness of the tube 3 leads to the loss of the solenod characteristics because the tube 3 contains nonmagnetic material. It also disadvantageously results in a greater tube length. The tube 3 is welded to the closure member 4 and the stationary core 2 at 5 and 6, respectively. The armature 1 has a pin 9 secured thereto. The closure member 4 has a push pin 12.
In view of the aforementioned disadvantages, it has been proposed to provide a tube enclosing the stationary core and form the outer surface of the tube with a grip portion for a tool, as disclosed in an outstanding application of the assignee of this invention which has not yet been published. But this arrangement is still unsatisfactory because there is every possibility that a gap may be formed between the stationary core and the yoke and impair the performance of the solenoid.
It is an object of this invention to solve the aforementioned problems and provide a small AC or DC solenoid having a long life and making only low noise in which any unnecessary increase of the force of attraction for the armature in a position closer to the stationary core is prevented by reducing the effective cross-sectional area of a magnetic path extending through the yoke and the armature, namely, by reducing the number of the lines of effectively acting magnetic flux at one end of the armature.
It is another object of this invention to provide a small AC or DC solenoid having a long life and making only low noise in which that end of an armature which is closer to a stationary core is provided with a means forming a section of leakage flux in which the armature and the stationary core have axially overlapping portions to thereby prevent any unnecessary increase of the force of attraction acting on the armature in a position closer to the stationary core.
It is a further object of this invention to provide an oil-immersed AC or DC solenoid which has eliminated the drawbacks of the prior art, and can be tightly secured to a valve body without impairing the solenoid characteristics or deforming the armature tube.
These and other objects are accomplished by an AC or DC solenoid of the armature in tube type for use with hydraulic or air valves or other machines, which solenoid has an armature, a stationary core, an armature tube secured to the stationary core in an oil-tight fashion and enclosing the armature axially slidably therein, and a yoke and a coil energizing the armature and the stationary core, and further comprises a means provided at an end of the armature for reducing the effective cross-sectional area of a magnetic path through the yoke and the armature when the armature has approached the stationary core more closely than a predetermined distance.
According to another aspect of this invention, in an AC or DC solenoid having an armature, a stationary core, and a yoke and a coil energizing the armature and the stationary core, there is provided an improvement comprising a section of leakage flux formed in an end of the armature closer to the stationary core and in which the armature and the stationary core axially overlap each other.
According to a further aspect of this invention, there is provided an AC or DC solenoid of the armature in tube type having a stationary core, an armature and an armature tube secured to the stationary core in an oil-tight fashion and enclosing the armature axially slidably, and further comprising a threaded portion provided on the outer wall of the stationary core at its end remote from the armature tube and engaged in a threaded hole formed in an adjacent end surface of the valve body, and a tool engaging means provided in a portion of the outer wall of the stationary core remote from the threaded portion thereof.